The discovery of high critical temperature (T.sub.c) ceramic superconductors has inspired an enormous interest in their application. Conventional niobium alloy superconductors such as NbTi must be cooled well below 10 K to achieve useful superconductivity. High T.sub.c superconductors, on the other hand, have T.sub.c s over 100 K. Due to the great expense of cryogenic refrigeration, the high T.sub.c materials could find much wider application in electronic and laboratory devices. Of particular interest are materials which have T.sub.c above 77 K, because this is the temperature of liquid nitrogen, a common and relatively inexpensive refrigerant.
Ceramic superconducting materials have not been used in many potential applications because they suffer from a number of shortcomings. The most severe problems with the ceramic superconductors are as follows:
1) They are brittle. They are not flexible and thus cannot be made into wires or other useful shapes. Cracks and boundaries between adjacent crystals severely limit supercurrent flow.
2) They are highly anisotropic. Supercurrents preferentially flow in certain directions with respect to the crystal lattice, reducing maximum supercurrent flow in randomly oriented multicrystalline pieces.
3) They are strong oxidizing agents. Many metals, such as copper, lead, tin, and niobium, are oxidized by contact with the ceramic superconductors, forming an insulating layer which impedes supercurrent flow. Only noble metals such as gold, silver, palladium and their alloys resist being oxidized by the ceramic superconductors.
A less severe undesirable feature of the high temperature ceramic superconductors is that they can lose their superconducting properties. The superconducting structure inside the ceramics has an abundance of oxygen atoms which are necessary for superconductivity. Heating, grinding, etching, or prolonged exposure to ambient atmosphere or vacuum liberates the oxygen and destroys superconductivity. Oxygen content can be restored by annealing the ceramic in an atmosphere with a partial pressure of oxygen.
The A15 superconductors such as Nb.sub.3 Sn are also brittle materials (although they are not anisotropic and relatively nonreactive). Their poor mechanical properties has precluded their use in many applications requiring ductility such as wires. This is unfortunate because they generally have superior superconducting properties such as high T.sub.c s and high critical magnetic fields.
To overcome the brittleness and anisotropy of the high T.sub.c ceramics, the superconducting proximity effect has been exploited in a number of prior art inventions.